gpio/sifive: Add DT documentation for SiFive GPIO

DT json-schema for GPIO controller added.
Signed-off-by: Wesley W. Terpstra <wesley@sifive.com>
[Atish: Compatible string update]
Signed-off-by: Atish Patra <atish.patra@wdc.com>
Signed-off-by: Yash Shah <yash.shah@sifive.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1575976274-13487-5-git-send-email-yash.shah@sifive.com

Documentation/devicetree/bindings/gpio/sifive,gpio.yaml

+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/gpio/sifive,gpio.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: SiFive GPIO controller
+
+maintainers:
+  - Yash Shah <yash.shah@sifive.com>
+  - Paul Walmsley <paul.walmsley@sifive.com>
+
+properties:
+  compatible:
+    items:
+      - const: sifive,fu540-c000-gpio
+      - const: sifive,gpio0
+
+  reg:
+    maxItems: 1
+
+  interrupts:
+    description:
+      interrupt mapping one per GPIO. Maximum 16 GPIOs.
+    minItems: 1
+    maxItems: 16
+
+  interrupt-controller: true
+
+  "#interrupt-cells":
+    const: 2
+
+  clocks:
+    maxItems: 1
+
+  "#gpio-cells":
+    const: 2
+
+  gpio-controller: true
+
+required:
+  - compatible
+  - reg
+  - interrupts
+  - interrupt-controller
+  - "#interrupt-cells"
+  - clocks
+  - "#gpio-cells"
+  - gpio-controller
+
+additionalProperties: false
+
+examples:
+  - |
+      #include <dt-bindings/clock/sifive-fu540-prci.h>
+      gpio@10060000 {
+        compatible = "sifive,fu540-c000-gpio", "sifive,gpio0";
+        interrupt-parent = <&plic>;
+        interrupts = <7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22>;
+        reg = <0x0 0x10060000 0x0 0x1000>;
+        clocks = <&tlclk PRCI_CLK_TLCLK>;
+        gpio-controller;
+        #gpio-cells = <2>;
+        interrupt-controller;
+        #interrupt-cells = <2>;
+      };
+
+...